Polymer composition, a process for the production thereof and films prepared thereof

ABSTRACT

This invention concerns process for producing polyethylene compositions, films prepared thereof and process for the preparation of the films. The process comprises subjecting ethylene, optionally hydrogen and comonomers to polymerization or copolymerization reactions in a multistage polymerization. At least one polymerization stage is conducted essentially in the absence of hydrogen. The polymerization reactions are carried out in the presence of a single-site catalyst capable of forming a composition comprising a low molecular weight component with MFR 2  of at least 10 g/10 min and a density higher than the density of the composition and a high molecular weight component. The melt flow rate of the composition is in the range MFR 2 =0.1-5.0 g/10 min and the density of the composition is 915-960 kg/m 3 . The invention makes it possible to produce polyethylene compositions for manufacturing films with a good balance between optical and mechanical properties.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/FI00/00005 which has an Internationalfiling date of Jan. 4, 2001, which designated the United States ofAmerica and was published in English.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a process for the production of polymercompositions. In addition, the present invention concerns films preparedof bimodal polymer compositions obtained by the present process. Inparticular, the present invention relates to bimodal films having animproved balance between the optical and mechanical properties and agood processability.

2. Description of Related Art

The processability on a film blowing line as well as the physicalproperties of the final film depend largely on the polymer structure,especially on the molecular weight distribution (MWD). If the polymer isbimodal, i.e., if the MWD is broad, the polymer can be expected toexhibit a good processability. Other important properties, whichnaturally depend on the application the polymer material is used in,comprise optical properties (i.e., the film should be clear and glossy)and mechanical properties.

Conventionally, linear low density polyethylene (PE-LLD) having abimodal molecular weight distribution is produced by polymerization inthe presence of Ziegler catalysts in two cascaded reactors. Similarly,high density polyethylene (PE-HD) having a bimodal MWD has been producedby polymerization in the presence of Ziegler catalysts in two reactorsin series.

Medium density polyethylene (PE-MD) for blown film is typically suppliedby unimodal Cr-based products. These materials are extensively used incoextruded films as a stiffness-improvement, but give relatively lesscontribution to other physical properties like impact required bypackaging.

The use of a metallocene catalyst in a two-stage polymerization processis known from EP-A-447035 and EP-A-881237. Bimodal polyethylene for filmis known from e.g. EP-A-605952, EP-A-691353. EP-A-691367 andWO-A-9618662.

EP-A-447035 discloses an ethylene polymer composition having a densityof 860-940 kg/m³ and an intrinsic viscosity of 1-6 dl/g, which wouldmean with a rough calculation a MFR₂ in a range of approximately 0,04-60g/10 min. The composition has been produced using a catalyst comprisinga ligand that has a cycloalkadienyl skeleton and an organoaluminumoxy-compound. The publication refers to the reduced fraction of polymersoluble in n-decane in the resins produced according to the invention.It further states that when the fraction of such polymer is low thepolymer has excellent anti-blocking properties.

In addition, the publication states that the target has been to producematerials having the clarity of the unimodal metallocene-based resin buta superior processability. The publication does not, however, disclosewhether or not the good clarity and improved processability actuallywere achieved. The melt flow rate region disclosed in the publication isconsiderably large, which seems to indicate that it was not clear whatcombination of density and intrinsic viscosity would result in bestprocessability and best clarity. Comparative examples shall show thatmany materials satisfying the definitions of EP-A447035 are notappropriate for producing films with a good combination of optical andmechanical properties.

EP-A-605952 discloses a polymer composition comprising two differentethylene polymers, which have been obtained by using a catalystcomprising at least two different metallocene compounds. This type ofcatalyst is sometimes referred to as dual site catalyst. The publicationdiscloses that the two ethylene polymers may be polymerized separatelyand blended in an extruder, or the polymers may be dissolved and thencombined. The polymers may also be produced in a two stagepolymerization. The composition can be used to prepare films.

The examples show that the compositions where a dual site catalyst wasused to prepare the polymer components produced films with good opticalproperties, high impact strength and good processability (ormouldability). Comparative examples 1 and 2 further disclose thatcompositions where a single component catalyst was used to prepare thepolymer components, produced films with inferior optical and mechanicalproperties and poor processability.

EP-A-881237 discloses a two-stage process to produce ethylene polymers,wherein a metallocene catalyst based on a tetrahydroindenyl compound wasused in a two-stage polymerization process. The document furtherdiscloses that the density of the polymer may range from 900 to 970kg/m³ and the high load melt index (MFR₂₁) from 0.1 to 45000 g/10 min.The examples disclose that the polymer was produced in loop and CSTRreactors. The materials disclosed in the examples had a density between938 and 955 kg/M³ and a melt index MFR₂ between 0.18 and 1.2 g/10 min.The use of the polymer was not disclosed and no practical examplesconcerning the use of the polymer were given.

EP-A-691367 discloses a film extruded from an in-situ blend of ethylenepolymers prepared using Ziegler-Natta catalysts. The publication statesthat the resulting resins have a high mechanical strength. It is alsostated in the publication that the film has a good processability and alow blocking tendency. The optical properties or gel level are notreferred to. It is, however, known in the art that films made of suchblends tend to be hazy.

EP-A-691353 discloses a process for preparing an in-situ blend giving alow gel level film. The process comprises polymerizing ethylene (withcomonomer) in a cascade of gas phase reactors using a Ziegler-Nattacatalyst. The publication further discloses that the resulting materialhas a good processability in blown film line.

WO-A-9618662 discloses a process for producing both high density andlinear low density film material. The process comprises a cascade of aloop and a gas phase reactor. In the process, also a prepolymerizer isincluded. The publication also states that metallocene catalysts may beused in the process. However, it does not reveal the purpose for using ametallocene catalyst nor the advantages of it.

Thus, as apparent from the above, the available materials for films givelimited alternatives in terms of a balance between clarity andmechanical properties.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved processfor producing suitable polyethylene materials for the production offilms.

It is another object of the present invention to provide novel polymercompositions for film-making.

It is a further object of the present invention to eliminate theproblems of the prior art and to provide novel polymer films.

These and other objects, together with the advantages thereof over knownprocesses and products, which shall become apparent from thespecification which follows, are accomplished by the invention ashereinafter described and claimed.

The present invention is based on the provision of bimodal polyethylenecompositions comprising

a first (low molecular weight) component with MFR₂ at least 10 g/10 minand a density higher than the density of the composition,

at least one other component,

said composition having a melt flow rate in the range MFR₂=0.1-5.0 g/10min and a density of 905-960 kg/m³.

According to one embodiment, the present invention provides a bimodalpolyethylene compositions comprising

a first (low molecular weight) component with MFR₂ at least 10 g/10 minand a density higher than the density of the composition,

at least one other component,

said composition having a melt flow rate in the range MFR₂=0.1-5.0 g/10min and a density of 915-960 kg/m³.

The composition is further characterized by a shear thinning index (SHI)of 3-20, viscosity of 5000-25000 Pas and storage modulus G′_(5kPa) of800-2500 Pa. It can be used for manufacturing polyethyiene films. Thefilms according to the invention exhibit excellent balance betweenoptical and mechanical properties.

The composition for polyethylene films can be produced by polymerizingor copolymerizing ethylene in a reactor cascade formed by at least tworeactors in the presence of a metallocene catalyst capable of producinga high molecular weight polymer in the absence of hydrogen. The problemin using metallocene catalysts in the production of bimodal polyethylenehas been that either they have not been able to produce a high enoughmolecular weight necessary in this kind of a process or their activityis too low to ensure an economic operation of such a process. Especiallydifficult has been to activate hafnium metal containing metallocenecatalysts on a carrier. Now it has surprisingly been found that thecatalysts according to the present invention, which are described indetail later on, are able to fulfil all the objectives and thus aresuitable for the production of bimodal polyethylene in a processinvolving heterogeneous catalysis.

More specifically, the process according to the present invention ismainly characterised by what is stated in the characterising part ofclaim 1.

The present film-making process is characterised by what is stated inthe characterising part of claim 13.

The present polyethylene film is characterised by what is stated in thecharacterising part of claim 18.

Considerable advantages are obtained by means of the present invention.The present process enables the preparation of resins having goodoptical properties (high gloss), processability similar to existingZiegler-Natta materials and superior to that of unimodal metallocenematerials (higher shear thinning index) and mechanical properties whichare comparable to existing commercial materials (puncture, elongation)by using a metallocene catalyst.

The novel composition for films may be used for producing both blown andcast films. It is, however, particularly suitable for film blowing, withthe improved optical properties.

The particular advantage of applying the invention in high density filmproduction is that a material with very good optical properties can beobtained. This has not been possible using conventional methods whichproduce hazy films. Still, as will later be shown in the examples, themechanical properties are on a good level.

Medium density films having a density range between 930-940 kg/m³ and agood clarity can be produced using conventional methods also. However,the invention allows for the combination of the good optical propertieswith good mechanical properties and a good processability.

As in medium density area, also in the low density area films havinggood optical properties have been known. But again, the combination ofgood optical properties and good mechanical properties is a new feature.

Next, the invention will be more closely examined with the aid of thefollowing detailed description and with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents the heat sealing behaviour for different materials.

FIG. 2 presents the heat sealing properties for different materials.

DETAILED DESCRIPTION OF THE INVENTION Definitions

For the purpose of the present invention, “slurry reactor” designatesany reactor operating in slurry, in which reactor the polymer forms inparticulate form. As examples of suitable reactors can be mentioned acontinuous stirred tank reactor, a batch-wise operating stirred tankreactor or a loop reactor. According to a preferred embodiment theslurry reactor comprises a loop reactor.

By “gas phase reactor” is meant any mechanically mixed or fluidized bedreactor. Preferably the gas phase reactor comprises a mechanicallyagitated fluidized bed reactor with gas velocities of at least 0.2m/sec.

By “melt flow rate” or abbreviated “MFR” is meant the weight of apolymer extruded through a standard cylindrical die at a standardtemperature in a laboratory rheometer carrying a standard piston andload. MFR is a measure of the melt viscosity of a polymer and hence alsoof its molecular weight. The abbreviation “MFR” is generally providedwith a numerical subindex indicating the load of the piston in the test.Thus, e.g., MFR₂ designates a 2.16 kg load. MFR can be determined using,e.g., by one of the following tests: ISO 1133 C4, ASTM D 1238 and DIN53735.

In the present invention, the rheological properties of polymers havebeen determined using Rheometrics RDA II Dynamic Rheometer. Themeasurements have been carried out at 190° C. under nitrogen atmosphere.The measurements give storage modulus (G′) and loss modulus (G″)together with an absolute value of complex viscosity (η*) as a functionof frequency (ω) or absolute value of complex modulus (G*).

η*=(G′ ² +G″ ²)/ω

G*=(G′ ² +G″ ²)

According to Cox-Merz rule the complex viscosity function, η*(ω) is thesame as conventional viscosity function (i.e., viscosity as a functionof shear rate), if frequency is expressed in rad/s. If this empiricequation is valid, then the absolute value of complex moduluscorresponds to shear stress in conventional (i.e., steady state)viscosity measurements. It can thus be concluded that also the functionη*(G*) accounts for viscosity as a function of shear stress.

In the present method, viscosily at a low shear stress or η* at a low G*(which serve as an approximation of zero viscosity), is used as ameasure of average molecular weight. On the other hand, shear thinning,i.e. the decrease of viscosity with G*, gets the more pronounced thebroader the molecular weight distribution is. This property can beapproximated by defining a shear thinning index. SHI, as a ratio ofviscosities at two different shear stresses. In the examples below theshear stresses (or G*) of 0 and 100 kPa have been used. Thus:

SHI _(0/100)=η*₀/η*₁₀₀

where

η*₀ is the zero shear rate viscosity

η*₁₀₀ is complex viscosity at G*=100 kPa

As already mentioned, the storage modulus function, G′(ω), and the lossmodulus function, G″(ω), are obtained as primary functions from dynamicmeasurements. The value of the storage modulus at a specific value ofloss modulus increases with the broadness of molecular weightdistribution. However, this quantity is highly dependent on the shape ofthe molecular weight distribution of the polymer. In the examples avalue of G′ at G″=5 kPa is used.

It should be noted that the viscosity, shear thinning index and storagemodulus values are used as measures of average molecular weight andmolecular weight distribution. The reason for using these valuesobtained from rheology, rather than the values of molecular weights andmolecular weight distribution obtained directly from size exclusionchromatography, is that the size exclusion chromatography can berelatively insensitive to the high molecular weight end of the molecularweight distribution. The information of this high molecular weight endis important, however, because the processability of the polymer andmany properties, like mechanical and optical properties aresignificantly influenced by the high molecular weight end. Rheology, onthe other hand, is sensitive to the high molecular weight end of themolecular weight distribution, and for this reason rheology was used tocharacterise the polymer.

Melt fracture is detected in the polymer film as small, repeated bandswhich occur continuously and affect the optical properties of the film.In defining the degree of melt fracture, terms “yes”, “some”. “slight”,“very slight”, “no” are used. If the grade of melt fracture is “yes”,then the bands are clearly visible, and the film is no longertransparent. If there is “no” melt fracture then, as is obvious, thematerial does not exhibit such feature at all. In the case of “some”,“slight” and “very slight”, “some” means that the melt fracture isvisible and hinders the transparency a little, “slight” means that themelt fracture is somewhat visible but does not affect the opticalproperties of the material and “very slight” means that the meltfracture can hardly be seen. Of these grades “some”, “slight” and “veryslight” can be classified as non-significant amount of melt fracture.

POLYMERIZATION PROCESS

To produce the polymer compositions according to the invention, ethyleneis polymerized in the presence of a metallocene catalyst at elevatedtemperature and pressure. Polymerization is carried out in a series ofpolymerization reactors selected from the group of slurry and gas phasereactors. In the following, the reactor system comprises one loopreactor (referred to as “the first reactor”) and one gas phase reactor(referred to as “the second reactor”), in that order.

However, it should be understood that the reactor system can comprisethe reactors in any number and order. It is also possible to conduct theprocess in two or more gas phase reactors.

The high molecular weight portion and the low or medium molecular weightportion of the product can be prepared in any order in the reactors. Aseparation stage is normally needed between the reactors to prevent thecarryover of reactants from the first polymerization stage into thesecond one. The first stage is typically carried out using an inertreaction medium.

The catalyst used in the polymerization process is a single sitecatalyst. According to a preferred embodiment, no fresh catalyst isadded to the second polymerization stage. The catalyst should produce arelatively narrow molecular weight distribution and comonomerdistribution. Additionally, a very important feature of the catalyst isthat it should be able to produce a high enough molecular weight so thatgood mechanical properties and good processability are obtained. Thus,the catalyst should be able to produce a weight average molecular weightof at least 250000 g/mol, preferably at least 300000 g/mol at theconditions present in the polymerization stage where the high molecularweight component is produced. Some metallocene catalysts, like thosebased on a bis-(n-butyl cyclopentadienyl)zirconium dichloride complexand disclosed in FI-A-934917 are not able to produce a high enoughmolecular weight polyethylene and their usefulness in bimodalpolymerization is limited. An example of this is shown in EP-A-605952,where a similar type of catalyst was used in a two-stage polymerizationprocess in Comparative Examples 1 and 2, and bimodal resins having poorprocessability were obtained. It has been found that some metallocenecatalysts are able to produce a high enough molecular weight. Oneexample of such catalysts is the one disclosed in FI-A-934917 and whichis based on the complex having the general formula

(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂)

wherein

X₁ and X₂ are either same or different and are selected from a groupcontaining halogen, methyl, benzyl, amido or hydrogen,

Hf is hafnium,

Cp is a cyclopentadienyl group, and

R₁ and R₂ are either the same or different and are either linear orbranched hydrocarbyl groups containing 1-10 carbon atoms.

According to one embodiment of the present invention, a catalyst basedon an active complex having the above formula is used. According to thisembodiment, however, X₁ and X₂ are either same or different and areselected from a group containing halogen, methyl, benzyl or hydrogen.The other species in the formula are as defined above.

Especially suitable complexes of the kind described above arebis-(n-butyl cyclopentadienyl) hafnium dihalides.

Another group of suitable complexes are the siloxy-substituted bridgedbis-indenyl zirconium dihalides, which are disclosed in FI-A-960437.

According to one embodiment of the invention, X₁ and X₂ are selected sothat one of them is halogen, preferably chlorine, and the other is amethyl, amido or benzyl group or a hydrogen atom, preferably an amidogroup. For example, in the dihalide complexes described above, it ispossible to replace one of the halogens by a methyl, amido or benzylgroup or by a hydrogen atom. One example is thus a catalyst having anactive complex containing chlorine and an amido group.

These catalysts are typically supported on a solid carrier, but they mayalso be used as unsupported. The carrier is typically inorganic, andsuitable materials comprise, e.g., silica (preferred), silica-alumina,alumina, magnesium oxide, titanium oxide, zirconium oxide and magnesiumsilicate (cf. also FI-A-934917). The catalysts are normally usedtogether with an aluminumoxane cocatalyst. Suitable cocatalysts are,e.g., methylaluminumoxane (MAO), tetraisobutylaluminumoxane (TIBAO) andhexaisobutylaluminumoxane (HIBAO). The cocatalyst is preferablysupported on the carrier, typically together with the catalyst complex,although the cocatalyst may optionally be fed into the reactorseparately.

A catalyst based on bis-(n-butyl cyclopentadienyl) hafnium dihalidecomplex supported on a silica or a silica-alumina carrier together witha methylaluminoxane cocatalyst is suitable to be run in a processincluding a loop rector and a gas phase reactor. Especially suitable isa catalyst based on bis-(n-butyl cyclopentadienyl) hafnium dichloride.Both the complex and the cocatalyst are supported on the carrier. Thethus obtained catalyst is then fed into the reactor. The catalyst iseasy to feed and the polymer settles well in the loop reactor. Thismakes the loop reactor operation easy. In the gas phase reactor thecatalyst is able to produce a sufficiently high molecular weightmaterial. This is essential to obtain the required processability on thefilm line and good mechanical properties of the film.

In every polymerization step it is possible to use also comonomersselected from the group of C₃₋₁₈ olefins, preferably C₄₋₁₀ olefins, suchas 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene,1-octene, 1-nonene and 1-decene as well as mixtures thereof and dienes,such as 1.5-hexadiene and 1,9-decadiene.

In addition to the actual polymerization reactors used for producing thebimodal ethylene homo- or copolymer, the polymerization reaction systemcan also include a number of additional reactors, such as prereactors.The prereactors include any reactor for prepolymerizing the catalyst andfor modifying the olefinic feed, if necessary. All reactors of thereactor system are preferably arranged in series (in a cascade).

The polymerization steps may be performed in the most convenient order.Thus, it is possible to polymerize the low molecular weight component inthe first step of the process and the high molecular weight component inthe second step. It is also possible to perform the steps in a reversedorder, i.e., to polymerize the high molecular weight component in thefirst stage and the low molecular weight component in the second stage.If the first stage involves a slurry polymerization, it is preferred toproduce the low molecular weight component in that stage to avoidproblems due to the solubility of the polymer.

According to a preferred embodiment of the invention, the polymerizationcomprises the steps of

subjecting ethylene and, optionally, hydrogen and comonomers to a firstpolymerization or copolymerization reaction in the presence of asingle-site catalyst in a first reaction zone or reactor to produce apolymer having a MFR of 10 g/10 min or more,

recovering the first polymerization product from the first reactionzone,

feeding the first polymerization product to a second reaction zone orreactor,

feeding additional ethylene and, optionally, comonomers to the secondreaction zone,

subjecting the additional ethylene and optionally additional monomer(s)and/or hydrogen to a second polymerization reaction in the presence ofthe single-site catalyst and the first polymerization product to producea second polymerization product having a MFR₂ of less than 5 g/10 min,and

recovering the combined polymerization product from the second reactionzone.

In the first step of the process, ethylene with the optionalcomonomer(s) is fed into the first polymerization reactor. Along withthese components is fed also hydrogen which functions as a molecularweight regulator. The amount of hydrogen depends on the desiredmolecular weight of the polymer. The catalyst may be fed to the reactortogether with the reagents or, preferably, by flushing with a diluent.

The polymerization medium typically comprises the monomer (i.e.ethylene) and/or a hydrocarbon, in particular, a light inert hydrocarbonsuch as propane, iso-butane, n-butane or isopentane. The fluid is eitherliquid or gaseous. In the case of a slurry reactor, in particular a loopreactor, the fluid is liquid and the suspension of polymer is circulatedcontinuously through the slurry reactor, whereby more suspension ofpolymer in particle form in a hydrocarbon medium or monomer will beproduced.

The conditions of the slurry reactor are selected so that 30-70 wt-%,preferably 40-60 wt-%, of the whole production is polymerized in theslurry reactor(s). The temperature is in the range of 40 to 110° C.,preferably in the range of 70 to 100° C. The reaction pressure is in therange of 25 to 100 bar, preferably 35 to 80 bar and the mole fraction ofethylene in the reaction mixture is typically 3-10% by mole. In order toproduce a polyethylene having a density in excess of 960 kg/m³, thepolymerization is preferably carried out at supercritical conditions attemperatures over 90° C. In slurry polymerization more than one reactorcan be used in series. In such a case the polymer suspension in areaction medium produced in the slurry reactor is fed without separationof inert components and monomers periodically or continuously to thefollowing slurry reactor, which acts at lower pressure than the previousslurry reactor.

The polymerization heat is removed by cooling the reactor with a coolingjacket. The residence time in the slurry reactor must be at least 10minutes, preferably 20-100 min for obtaining a sufficient degree ofpolymerization.

As discussed above, if a low molecular weight polyethylene is thedesired product, hydrogen is fed into the reactor. With a catalystaccording to the invention, a very small amount of hydrogen issufficient to produce a high MFR₂ polyethylene. Thus, an MFR₂ of 50-300g/10 min can be obtained with a hydrogen-to-ethylene feed ratio between0.1-0.5 kg of hydrogen/ton of ethylene. The hydrogen is typicallyconsumed in the reactor, so that it cannot be detected by analysis,e.g., by gas chromatography or only small amount of hydrogen is presentin the reaction mixture. Typically the molar ratio of hydrogen toethylene is between 0.4 and 1 mol/kmol, or 400-1000 ppm by volume.

After the first reaction zone the volatile components of the reactionmedium are evaporated. As a result of the evaporation, hydrogen isremoved from the product stream. The stream can be subjected to a secondpolymerization in the presence of additional ethylene to produce a highmolecular weight polymer.

The second reactor is preferably a Pas phase reactor, wherein ethyleneand preferably comonomers are polymerized in a gaseous reaction mediumin the presence of a single-site catalyst. If it is desirable to obtaina high molecular weight polymer, the polymerization is conductedessentially in the absence of hydrogen. The expression “essentially inthe absence of hydrogen” means, for the purposes of this invention, thatno additional hydrogen is fed to the reactor and the amount of hydrogenpresent in the reactor is typically less than 0.4 mol/kmol (400 ppm byvolume), preferably less than 0.3 mol/kmol (300 ppm by volume).According to one embodiment of the invention the amount of hydrogen isso small that it cannot be detected with analytical equipment commonlyused in this kind of application, e.g., gas chromatograph. The gas phasereactor can be an ordinary fluidized bed reactor, although other typesof gas phase reactors can be used. In a fluidized bed reactor, the bedconsists of the formed and growing polymer particles as well as stillactive catalyst come along with the polymer fraction. The bed is kept ina fluidized state by introducing gaseous components, for instancemonomer and optionally comonomer(s) on such a flow rate that will makethe particles act as a fluid. The fluidizing gas can contain also inertcarrier gases, like nitrogen and propane and also hydrogen as amolecular weight modifier. The fluidized gas phase reactor can beequipped with a mechanical mixer.

The gas phase reactor used can be operated in the temperature range of50 to 115° C., preferably between 60 and 110° C. and the reactionpressure between 10 and 40 bar and the partial pressure of ethylenebetween 1 and 20 bar, preferably 5-10 bar.

The production split between the high molecular weight polymerizationreactor and the low molecular weight polymerization reactor is30-70:70-30. Preferably, 30 to 70 wt-%, in particular 40 to 60%, of theethylene homopolymer or copolymer is produced at conditions to provide apolymer having a MFR₂ of 10 g/10 min or more and constituting the lowmolecular weight portion of the polymer, and 70 to 30 wt-%, inparticular 60 to 40 wt-%, of the ethylene homopolymer or preferablycopolymer is produced at conditions to provide a polymer having a MFR₂of less than 5 g/10 min, in particular about 0.4 to 5 g/10 min andconstituting the high molecular weight portion of the polymer. Thedensity of the low molecular weight portion is preferably 940-975 kg/m³and the density of the final polymer is preferably 915 to 960 kg/m³.

The present polymers and copolymers of ethylene can be blended andoptionally compounded with additives and adjuvants conventionally usedin the art. Thus, suitable additives include antistatic agents, flameretardants, light and heat stabilizers, pigments, processing aids andcarbon black. Fillers such as chalk, talc and mica can also be used.Note, however, that while processing aids, such as fluoroelastomers, canbe added to the polymer composition, they are not needed to ensure agood processability. The compositions according to the present inventioncan easily be processed to a film without the addition of processingaids.

THE POLYMER COMPOSITION

The present invention concerns also polyethylene compositions having abimodal molecular weight distribution and comprising a high molecularweight portion and a low molecular weight portion. The MFR₂ of the lowmolecular weight portion of the composition is at least 10 g/10 min andthe density of the low molecular weight portion is higher than thedensity of the composition.

The polyethylene composition comprises, according to the invention, 30to 70 wt-%, preferably 40 to 60 wt-% of a high molecular weight portion,and 70 to 30 wt-%, preferably 60 to 40 wt-% of a low molecular weightportion. The melt flow rate the composition is in the range from aboutMFR₂=0.1-5.0 g/10 min, preferably 0.4-3.0 g/10 min.

According to one embodiment of the invention 30 to 70 wt-%, inparticular 40 to 60 wt-% of the composition is formed of an ethylenepolymer having a MFR₂ of 10 g/10 min or more and 70 to 30 wt-%, inparticular 60 to 40 wt-%, of the composition is formed of an polymerhaving a MFR₂ of less than 5 g/10 min.

The density of the polymer product is about 905 to 960 kg/m³, inparticular 915 to 960 kg/m³. The composition is further characterized bya shear thinning index (SHI_(0/100)) of 3-20, preferably of 3.5-15, azero shear rate viscosity of 5000-25000 Pas, preferably of 8000-20000Pas, and a storage modulus G′_(5kPa) of 800-2500 Pa.

The density of the polymer and the density and melt flow rate of the lowmolecular weight component correlate preferably as presented in thefollowing:

If the density of the composition is between 940-960 kg/m³, the MFR₂ ofthe low molecular weight component is higher than 50 g/10 min,preferably between 50-1000 g/10 min and the density is higher than 965kg/m³.

If the density of the composition is in the medium density area, i.e.,between 930-940 kg/m³, the low molecular weight component preferably hasan MFR₂ between 20-1000 g/10 min and a density between 940-975 kg/m³.

If the density of the composition is low, between 905-930 kg/m³, inparticular 915-930 kg/m³ the low molecular weight component preferablyhas an MFR₂ between 10-500 g/10 min and density between 925-965 kg/m³,in particular 935-965 kg/m³.

As specific examples of preferred embodiments, the following ispresented:

A composition with a melt flow rate MFR₂ lower than 5 g/10 min, densityof 940-960 kg/m³, a zero shear rate viscosity of 5000-21 000 Pas, shearthinning index (SHI_(0/100)) of 5-20 and a storage modulus G′_(5kPa) of1000-2500 Pa.

A composition with a melt flow rate MFR₂ lower than 5 g/10 min, adensity of 930-940 kg/m³, a zero shear rate viscosity of 5000-25000 Pas,shear thinning index (SHI_(0/100)) of 3-15 and storage modulus G′_(5kPa)of 800-2100 Pa.

A composition with a melt flow rate MFR₂ in the range of 0.4-5 g/10 min,a density of 915-930 kg/m³, a zero shear rate viscosity of 8000-20000Pas, shear thinning index (SHI_(0/100)) of 5-20 and a storage modulusG′_(5kPa) of 800-2000 Pa.

A composition with a melt flow rate MFR₂ in the range of 0.4-5 g/10 min,a density of 905-930 kg/m³, a zero shear rate viscosity of 8000-20000Pas, shear thinning index (SHI_(0/100)) of 3-20 and a storage modulusG′_(5kPa) of 800-2000 Pa.

Due to the good mechanical properties in combination with excellentoptical properties, the present polyethylene compositions can be blownor extruded to films. The composition is particularly suitable for filmblowing. The polymer is fed, typically in the form of powder or pellets,optionally together with additives, to a film blowing or extrudingdevice.

According to one embodiment of the invention the film exhibiting theoptical and mechanical properties described below is produced by blowingor extruding a polymer composition comprising

a low molecular weight component with MFR₂ of at least 10 g/10 min and adensity higher than the density of the composition,

a high molecular weight component,

said composition having a melt flow rate in the range MFR₂=0.1-5.0 g/10min, a density of 905-960 kg/m³, a zero shear rate viscosity of5000-25000 Pas, shear thinning index (SHI) of 3-20 and a storage modulusG′_(5kPa) of 800-2500 Pa.

According to another embodiment of the invention the film exhibiting theoptical and mechanical properties described below is produced by blowingor extruding a polymer composition comprising

a low molecular weight component with MFR₂ of at least 10 g/10 min and adensity higher than the density of the composition,

a high molecular weight component,

said composition having a melt flow rate in the range MFR₂=0.1-5.0 g/10min, a density of 915-960 kg/m³, a zero shear rate viscosity of5000-25000 Pas, shear thinning index (SHI) of 3-20 and a storage modulusG′_(5kPa) of 800-2500 Pa.

The thickness of the film is about 10-300 μm, preferably 20-100 μm, morepreferably 30-100 μm and in particular 30-80 μm. The films producedgenerally exhibit the following features:

haze less than 20%

gloss higher than 70%

dart drop higher than 150 g, and

non-significant melt fracture.

Thus, the film is glossy, clear and very well processable. Preferably,the gloss is higher than 80% and the film exhibits haze less than 15%.The non-significant melt fracture means that although traces of meltfracture may be detected in the film, it does not disturb the visualappearance of the film. What is more, the absence of melt fracture hasbeen obtained without adding processing aid additives, such asfluoroelastomers, to the polymer.

The present polyethylene compositions can also be used to produce heatsealable films, since compositions combine the excellent sealingproperties of unimodal metallocene based materials with theprocessability of the Ziegler materials.

Description of Analytical Methods

Laboratory Polymerization

Polymerization in a laboratory reactor is carried out as follows.

The first stage (slurry) is carried out using isobutane as a medium. Thesecond stage (gas phase) is run as a stirred bed technique in asemibatch mode after the evaporation of the medium by the same reactoras at the first stage.

Bimodal metallocene PE materials are produced using two stagepolymerization technique. Two different catalysts are employed in thepolymerizations. In the first stage slurry polymerization low molecularweight PE having controlled MFR and density is made. This is done bycontinuous hydrogen and 1-hexene introduction to the reactor. In thesecond stage (gas phase) high molecular weight portion is made bycontrolling the density values of the end composition with 1-hexeneaddition. The MFR is held constant because hydrogen was not present. Inboth stages the hydrogen/ethylene ratios are calculated as mol/kmol.

It is also possible to feed the materials first to a pre-mixing chamber,PMC. In the PMC the materials are mixed continuously with, e.g., apaddle stirrer. The feed of the first reactor can also consist of thereaction mixture from a previous reactor, if any, together with addedfresh monomer, optional hydrogen and/or comonomer and additionalcatalyst.

The polymerization temperature is 80° C. in both stages. The partialpressure of ethylene is 56-6.0 bar in the slurry polymerization and 5-10bar in the gas phase polymerization.

The polymerization rate is followed by recording the ethyleneconsumption. This is also used in determination of the split (ratiobetween high molecular eight and low molecular weight materials).

After the polymerization is completed, the polymer is recovered, driedand analysed.

Laboratory Material Compounding

Powder was fed first to the compounding unit, which was a twin screwBrabender DSK42/7, screw diameter D=42 mm and screw length/diameterratio L/D=7. Nominal output range of this unit was 1200-4800 g/h, but ithas been running succesfully at 500 g/h. Screws were counter rotated.

After the compounding unit the polymer melt was fed to the extrusionunit, which was Brabender single screw extruder, D=19 mm and L/D=25.Feed was connected to the gas ventilation point of the extruder, locatedso that 40% of extruder screw length was filled, i.e. in effective use.Extruder generated enough pressure for 100 mm cast film extrusion.

Gloss

Gloss is measured according to ASTM D 2457v.

Haze

Haze is measured according to ASTM 1003.

Dart Drop

Dart drop is measured using ISO 7765-1 method.

Puncture

A simple small scale puncture test is introduced. The film ismechanically clamped allowing circular testing area of diameter 50 mm.The film is then punctured by a striker (diameter 20 mm). The force andtravel to puncturing point are measured and the required energy iscalculated. Travelling speed of the striker is 200 mm/min.

Tensile Strength

The experiment is performed according to ISO 1184 method. The specimenis extended along its major axis at a constant speed. Normal 50 mm couldbe used as a distance between grips (gauge length) in film tensiletesting. 125 mm gauge length would be required for tensile modulusmeasurement so this was not possible for 100 mm cast film in transversedirection.

Tear Strength

Tear testing, is done according to ASTM 1922. Machine direction iseasier, as the thickness variation in critical test direction is bettercontrolled. Thickness varied more in transverse direction andoccasionally difficulties arise in taking the sample in a manner whichguarantees an even thickness for the critical testing area.

The invention is further illustrated with the aid of the followingexamples.

EXAMPLE 1

168 g of metallocene complex (bridged siloxy-substituted bis-indenylzirconium dichloride, according to a patent application FI 960437) and9. 67 kg of a 30% MAO solution supplied by Albemarle were combined and3.18 kg dry, purified toluene was added. The thus obtained complexsolution was added on 9 kg silica carrier SP9-243 by Grace havingaverage particle size of 20 microns, pore volume of 1.5-1.7 mm³ andspecific surface area of 350-400 m²/g. The complex was fed very slowlywith uniform spraying during 2 hours. Temperature was kept below 30° C.The mixture was allowed to react for 2 h after complex addition at 30°C.

The thus obtained catalyst was dried under nitrogen for 6 h at 75° C.temperature. After nitrogen drying the catalyst was further dried undervacuum for 10 h.

This catalyst is referred to as catalyst A in the subsequent examples.

EXAMPLE 2

134 g of a metallocene complex (TA02823 by Witco, n-butyldicyclopentadienyl hafnium dichloride containing 0.36% by weight Hf) and9.67 kg of a 30% MAO solution supplied by Albemarle were combined and3.18 kg dry, purified toluene was added. The thus obtained complexsolution was added on 17 kg silica carrier Sylopol 55 SJ by Grace. Thecomplex was fed very slowly with uniform spraying during 2 hours.Temperature was kept below 30° C. The mixture was allowed to react for 3h after complex addition at 30° C.

The thus obtained catalyst was dried under nitrogen for 6 h at 75° C.temperature. After nitrogen drying the catalyst was further dried undervacuum for 10 h.

This catalyst is referred to as catalyst B in the subsequent examples.

EXAMPLE 3

Catalyst, diluent, ethylene and optionally hydrogen and comonomer wereadded into a 10 dm³ laboratory polymerization reactor. A two stagepolymerization was performed as described above.

Polymerization runs were carried out in such conditions that materialsaccording to samples H1-H3, M1-M6 and L1-L6 in Table 1, Table 2 andTable 3 were obtained. Split presented in the tables is given as thefraction of the polymer produced in the first stage to the fraction ofpolymer produced in the second stage (i.e. 40/60 means that 40% of thematerial has been produced in the first stage and 60% in the secondstage).

The samples H2, M3, M4 and M5 were prepared in a reversed mode, i.e.,the high molecular weight component was prepared in the first stage andthe low molecular weight component in the second stage.

TABLE I HD materials, lab polymerization data Sample H1 H2 H3 CH1 CH2Catalyst B A A A A Density [kg,m³] 973 969 971 963 (1st stage) MFR₂[g/10 min] 109 20 49 118 (1st stage) Density [kg/m^(3]) 945 940 942 943944 MFR₂ [g/10 min] 3.5 0.8 1.2 13.3 10.3 Split 60/40 40/60 40/60 60/4060/40

TABLE 2 MD materials, lab polymerization data Sample M1 M2 M3 M4 M5 M6CM1 CM2 CM3 Catalyst B B B B A A B A A Density 970 962 951 959 976 973[kg/m³] (1^(st) stage) MFR₂ 25 261 98 30 193 88 [g/10 min] (1^(st)stage) Density 937 935 931 936 933 934 936 933 938 [kg/m³] MFR₂ 0.62 1.21.0 0.41 3.1 3.4 0.84 5.6 10.9 [g/10 min] Split 40/60 60/40 50/50 50/5050/50 50/50 50/50 50/50 50/50

TABLE 3 LLD materials, lab polymerization data Sample L1 L2 L3 L4 L5 L6CL1 CL2 CL3 Catalyst B B B B A A A B B Density 953 936 942 928 953 961948 957 948 [kg/m³] (1^(st) stage) MFR₂ 20 15 172 148 74 49 141 330[g/10 min] (1^(st) stage) Density 928 921 929 928 927 929 926 922 923[kg/m³] MFR₂ 1.2 1.3 0.68 0.47 3.4 3.1 7.8 0.57 0.33 [g/10 min] Split50/50 60/40 50/50 50/50 40/60 40/60 60/40 40/60 40/60

COMPARATIVE EXAMPLE 1

The polymerization runs were performed according to Example 3. Thesamples are denoted as CH1-CH2, CM1-CM3 and CL1-CL3 in Table 1, Table 2and Table 3.

The sample CM2 was prepared in a reversed mode of operation, i.e. thehigh molecular weight component was produced in the first stage and thelow molecular weight component in the second stage.

EXAMPLE 4

A pilot plant comprising a loop and a gas phase reactor was operated asfollows: The loop reactor was operated at 85° C. temperature and 60 barpressure. Propane, ethylene, hydrogen and 1-butene comonomer werecontinuously introduced into the reactor, together with thepolymerization catalyst, so that polymer production rate was about 25kg/h. Reactor conditions were such that polymer having propertiesaccording to Table 4 was produced. Polymer slurry was intermittentlydischarged from the reactor to a flash tank where the hydrocarbons wereseparated from the polymer. The polymer was introduced into a gas phasereactor, operated at 85° C. temperature and 20 bar pressure, whereadditional ethylene and 1-butene comonomer were added. Reactorconditions and polymer withdrawal rate were such that materialsaccording to Table 4 were obtained.

COMPARATIVE EXAMPLE 2

The procedure of Example 4 was repeated, except that a catalyst ofZiegler-Natta type, prepared according to Example 3 of EP-A-688794 wasused. The corresponding sample is denoted as CL4 in Table 4.

TABLE 4 LLD materials, pilot plant polymerization data Sample L7 L8 L9L12 L13 L14 L15¹⁾ L16¹⁾ CL4¹⁾ Catalyst B B B B A A A A Z Density [kg/m³941 938 940 937 937 936 929 931 937 (1^(st) stage) MFR₂ [g/10 min] 13080 130 93 119 71 150 260 1.4 (1^(st) stage) Density [kg/m³] 925 919 921915 920 920 915 918 920 MFR₂ [g/10 min] 0.75 0.42 0.70 0.82 1.5 1.5 1.21.4 1.2 Split 40/60 39/61 45/55 42/58 49/51 50/50 48/52 48/52 51/49[loop-%/gpr-%] ¹⁾Loop reactor operated at 75° C.

EXAMPLE 5

The materials of the Example 3 and Example 4 were analysed. The analysisdata is shown in Table 5, Table 6, Table 7 and Table 8.

TABLE 5 HD lab materials, analysis data Material H1 H2 H3 CH1 CH2Density [kg/m³] 945 941 942 943 944 MFR₂[g/10 min] 3.5 0.8 1.2 13.3 10.3η₀ [Pas] 8260 20000 27000 3700 5000 SHI_(0/100) 6.3 6.9 17 12 19G′_(5kP2) [Pa] 1400 1545 2220 1940 2170

TABLE 6 MD lab materials, analysis data Material M1 M2 M3 M4 M5 M6 CM1CM2 CM3 Density 937 935 931 936 933 934 936 933 938 [kg/m³] MFR₂ 0.621.2 1.0 0.41 3.1 3.4 0.84 5.6 10.9 [g/10 min] η₀ [Pas] 18000 9680 1110023300 7600 8500 18000 8000 4700 SHI_(0/100) 3.0 3.5 4.0 4.4 14 10 5.1 2015 G′_(5kPa) [Pa] 866 983 1020 1040 2050 1850 1190 1960 2130

TABLE 7 LLD lab materials, analysis data Material L1 L2 L3 L4 L5 L6 CL1CL2 CL3 Density 928 921 929 928 927 929 926 922 923 [kg/m²] MFR₂ 1.2 1.30.68 0.47 3.4 3.1 7.8 0.57 0.33 [g/10 min] η₀ [Pas] 11300 10800 1660019800 8000 9300 2400 23400 34500 SHI_(0/100) 3.4 5.3 4.1 3.1 10 11 6.03.7 3.8 G′_(5kPa) 913 1300 1020 854 1850 1920 1470 995 985 [Pa]

TABLE 8 LLD pilot materials, analysis data Material L7 L8 L9 L12 L13 L14L15 L16 CL4 Density [kg/m³] 925 919 921 915 920 920 915 918 920 MFR₂0.75 0.42 0.70 0.82 1.5 1.5 1.2 1.4 1.2 [g/10 min] η₀ [Pas] 11900 2010013700 10000 8340 7840 8940 9130 9940¹ SHI_(0/100) 3.4 34 3.7 4.0 6.0 5.07.0 9.0 4.62 G′_(5kPa) [Pa] 898 855 890 973 1360 1290 1440 1610 1560 ¹:Viscosity at 1 kPa shear stress ²: SHI_(1/100)

REFERENCE EXAMPLE 1

A number of commercially available materials were evaluated. The resultsare shown in Table 10. Samples RL1, RL2, RL6 and RL7 are commerciallyavailable unimodal metallocene materials. Sample RL3 is a commerciallyavailable unimodal material produced using a Ziegler-Natta catalyst.Sample RL4 is a commercially available bimodal material produced using aZiegler-Natta catalyst. Sample RH1 is a blend of unimodal metallocenematerial and a unimodal Ziegler-Natta material. Sample RL5 is a unimodalmetallocene material produced in a loop reactor.

TABLE 9 Reference materials Sample RL1 RL2 RL3 RL4 RH1 RL5 RL6 RL7Density 919 918 919 923 940 920 923 917 [kg/m³] MFR₂ 1.0 0.9 1.1 0.76¹2.6¹ 3.4 2.8 0.95 [g/10 min] η₀ [Pas] 9360 9400 13500 — 17900² 3760 271010900 SHI_(0/100) 3.4 3.2 5.2⁴ — 9.1³ 1.5 1.5 4.0 G′_(5kPa) [Pa] 1003965 1360 — 1850 306 325 1090 Notes: ¹: MFR₅ ²: Viscosity at 1 kPa shearstress ³: SHI_(1/100) ⁴: SHI_(0/50)

EXAMPLE 6

A cast film was made from high density samples H1-H4, CH1 and RH1according to the procedure presented above. The results are shown inTable 10.

The table shows that the materials prepared according to the inventionH1-H4 exhibit good (H1-H2) or acceptable (H3-H4) mechanical properties,as indicated by the high puncture values, combined with good opticalproperties, as indicated by the high gloss. While the comparativematerial CH1 and the reference material RH1 have equally good punctureresistance, they have a clearly inferior gloss compared to the inventivematerials.

TABLE 10 HD materials, lab cast film data Material H1 H2 H3 H4 CH1 RH1Film thickness 37 36 40 36 32 48 [μm] Tensile 50.0 47.0 25.0 30.0 46.054.6 strength MD [MPa] Tensile 26.0 35.0 22.0 22.0 30.0 29.2 strength TD[MPa] Puncture 0.9 1.4 0.6 0.5 0.9 1.4 energy [J] Puncture/ 0.024 0.0390.015 0.014 0.028 0.029 thickness Gloss 96 96 90 90 81 61

EXAMPLE 7

The procedure of Example 6 was repeated, but now the medium densitysamples M1-M6 and CM1-CM3 were used as starting materials. The resultsare shown in Table 11.

The table shows that the inventive materials again have a bettercombination of mechanical and optical properties than the comparativematerials. A look at Table 6 reveals that a too low molecular weight(low viscosity) results in poor mechanical properties (CM2). It alsoshows that if the molecular weight is too high (or the molecular weightdistribution too broad), the optical properties will suffer (CM1 andCM3).

TABLE 11 MD materials, lab cast film data Material M1 M2 M3 M4 M5 M6 CM1CM2 CM3 Thickness 34 40 41 37 38 34 35 28 30 [μm] TS MD [Mpa] 85 49 6267 34 40 56 30 33 TS TD [Mpa] 35 35 35 32 26 27 32 24 27 Puncture [J]2.1 1.3 1.3 1.1 1.1 1.2 0.9 0.5 0.8 Puncture/ 0.032 0.033 0.032 0.0300.029 0.035 0.026 0.018 0.027 Thickness Gloss 97 128 104 90 100 114 72112 84

EXAMPLE 8

The procedure of Example 6was repeated, but now the low density samplesL1-L6 and CL1-CL3 were used as starting materials. The results arepresented in Table 12.

TABLE 12 LLD materials, lab cast film data Material L1 L2 L3 L4 L5 L6CL1 CL2 CL3 Thickness 34 30 36 51 30 29 27 37 39 [μm] TS MD [Mpa] 58 4663 75 37 36 33 70 74 TS TD [Mpa] 36 28 32 43 32 30 26 42 44 Puncture [J]1.4 1.2 1.6 2.2 1.2 1.3 0.9 1.5 1.6 Puncture/ 0.041 0.040 0.044 0.0430.040 0.045 0.033 0.041 0.041 Thickness Gloss 137 96 101 108 97 98 10176 77

TABLE 13 Reference materials lab cast film data Material RL1 RL2 RL3 RL4RL5 RL6 Thickness [μm] 42 43 36 77 88 TS MD [MPa] 59 49 53 22 TS TD[MPa] 44 32 29 20 Tear MD [N] 0.15 5.1 3.5 Puncture [J] 1.8 2 1.6 1.14.0 5.0 Puncture/ 0.043 0.047 0.044 0.052 0.057 thickness Gloss 126 139134

EXAMPLE 9

Materials L7-L9, RL3 and RL4 were blown to film on a Windmöller &Hölscher film line using a 200 mm die and 2.3 mm die gap. The blow-upratio was 2.5:1. The frost-line height was 450 mm and the film thickness40 μm. The film data is shown in Table 14.

To improve processability, a fluoroelastomer was added into samples L7and L8 so that the concentration of the fluoroelastomer was 300 ppm. Thethus obtained samples are shown in Table 14 as L10 and L11,respectively.

TABLE 14 Blown film data Material L7 L8 L9 L10 L11 L12 L15 RL3 RL4 Melttemp., ° C. 258 267 257 254 263 CoF, inside 0.64 0.84 0.7 0.7 0.78 1.32Outside 0.68 0.82 0.78 0.74 0.89 2.06 TS MD, MPa 33 47 35 41 39 21 13 35TS TD, Mpa 36 41 37 41 46 26 25 30 Tear MD, N 3.1 2.9 3.4 2.7 2.5 3.02.3 2.1 Dart drop, g 171 1023 478 176 1203 830 178 103 350 Haze 17 16 1712 14 37 17 9 65 Gloss 83 72 72 99 104 32 65 111 15 Melt fracture SlightYes Some No Slight No No Some No

The table shows that the film made of the inventive material has almostsimilar optical properties (slightly higher haze and lower gloss) thanthe film made of unimodal Ziegler-Natta material (RL3) but significantlyimproved mechanical properties (higher dart drop and tear strength).Compared to a bimodal Ziegler-Natta material (RL4), the film made of theinventive material has significantly improved optical properties and atleast comparable mechanical properties. The processability (of which theabsence of melt fracture is a useful measure; alternatively, melttemperature also measures processability, high melt temperatureindicating poor processability) of the inventive materials is comparableto that of the unimodal Ziegler-Natta material but inferior to bimodalZiegler-Natta material. A unimodal metallocene based material was notrun in this experiment, but earlier experience has shown that this kindof material has reasonable optical properties combined with goodmechanical properties but it is very difficult to process.

EXAMPLE 10

Materials L7, RL4 and RL6 were put to a hot tack test to measure thesealability. Two film samples were pressed together at elevatedtemperature. The sealing time was 0.2 seconds, the lag time was 0.1seconds and the sealing pressure was 1 N/mm². The force required tobreak the seal was then measured. The data is shown in FIG. 1. Itindicates that the film made of inventive material L7 has a similar heatsealing behaviour as the one made of unimodal metallocene material RL6.Both these are superior to unimodal Ziegler material RL4 (higher force,as well as lower sealing temperature). The difference between thesealing temperatures of L7 and RL6 is due to the fact that L7 has ahigher density compared with RL6 and thus the sealing temperature ishigher.

While L7 has a sealing behaviour similar to that of RL6, itsprocessability is similar to RL4, which is superior to theprocessability of RL6. Thus, the invention combines the sealingperformance of the metallocene materials to the processability of theZiegler materials.

EXAMPLE 11

Materials L14 and L16 and a comparative material produced with a Zieglercatalyst, CL4, were blown to film on a Reifenhäuser film line using a150 mm die and 1.5 mm die gap. The blow-up ratio was 3.0:1. The filmthickness was 25 μm. The film data is shown in Table 15.

The table shows that the inventive materials L14 and L16 have superiormechanical properties (same tensile strength with a thinner film, andhigher dart drop with equal film thickness) compared with thecomparative material CL4. Also, the optical properties are slightlybetter.

TABLE 15 Blown film data Material L14 L16 CL4 TS MD, Mpa 39 37 32*  TSTD, Mpa 34 31 39*  Tear MD, N 1.4 1.4 Dart drop, g 118 131 69   Haze 6.77.7  9.8 Gloss 110 98 Melt fracture No No No *measured from 40 μm film

EXAMPLE 12

The procedure of Example 10 was repeated, except that the sealing timewas 0.5 seconds, the tag time 0.1 seconds, sealing pressure was 90 N fora specimen width 15 mm. The film thickness was 40 μm. The results areshown in FIG. 2. The samples that were tested were L12, L15 and RL3. Theresults indicated that RL3 had poor hot tack and it had a reasonableprocessability. L12 and L15 had a good processability, and L12 hadinteresting hot tack properties (low sealing temperature) while L15 hada reasonable hot tack.

What is claimed is:
 1. A process for producing polyethylene compositionscomprising the steps of subjecting ethylene and, optionally, hydrogenand/or comonomers to polymerization or copolymerization reactions in amultistage polymerization sequence of successive polymerization stages,operating at least one polymerization stage essentially in the absenceof hydrogen so as to prepare a high molecular weight polymer component,carrying out the polymerization reactions in the presence of asingle-site catalyst, the active complex of which is asiloxy-substituted bis-indenyl zirconium dihalide or has the generalformula (X₁)(X₂)Hf(Cp-R₁)(Cp-R₂),  (I)  wherein X₁ and X₂ are eithersame or different and are selected from a group containing halogen,methyl, benzyl, amido or hydrogen, Hf is hafnium, Cp is acyclopentadienyl group, and R₁ and R₂ are either the same of differentand are either linear or branched hydrocarbyl groups containing 1-10carbon atoms, to form a polyethylene composition comprising a lowmolecular weight average component with MFR₂ of at least 10 g/10 min anda density higher that the overall density of the composition, a highmolecular weight average component, said polyethylene composition havinga melt flow rate in the range MFR₂=0.1-5.0 g/10 min and a density of905-960 kg/m³.
 2. A process for producing polyethylene compositionscomprising the steps of subjecting ethylene and, optionally, hydrogenand/or comonomers to polymerization or copolymerization reactions in amultistage polymerization sequence of successive polymerization stages,operating at least one polymerization stage essentially in the absenceof hydrogen so as to prepare a high molecular weight polymer component,carrying out the polymerization reactions in the presence of asingle-site catalyst, the active complex of which is asiloxy-substituted bis-indenyl zirconium dihalide or has the generalformula (X₁)(X₂)Hf(Cp-R₁)(Cp-R₂),  (I)  wherein X₁ and X₂ are eithersame or different and are selected from a group containing halogen,methyl, benzyl or hydrogen, Hf is hafnimum, Cp is a cyclopentadienylgroup, and R₁ and R₂ are either the same or different and are eitherlinear or branched hydrocarbyl groups containing 1-10 carbon atoms,  toform a polyethylene composition comprising a low molecular weightaverage component with MFR₂ of at least 10 g/10 min and a density higherthat the overall density of the composition, a high molecular weightaverage component, said composition having a melt flow rate in the rangeMFR₂=0.1-5.0 g/10 min and a density of 915-960 kg/m³.
 3. The processaccording to claim 1 or 2, comprising subjecting ethylene and,optionally, hydrogen and/or comonomers to a first polymerization orcopolymerization reaction in the presence of the single site catalyst ina first reaction zone or reactor to produce a polymer having a MFR of 10g/10 min or more, recovering the first polymerization product from thefirst reaction zone, feeding the first polymerization product to asecond reaction zone or reactor, feeding additional ethylene and,optionally, comonomers to the second reaction zone, subjecting theadditional ethylene and optionally additional monomer(s) to a secondpolymerization reaction in the presence of the single-site catalyst andthe first polymerization product to produce a second polymerizationproduct having a MFR₂ of less than 5 g/10 min, and recovering thecombined polymerization product from the second reaction zone.
 4. Theprocess according to claim 1, wherein the active complex of the catalystis bis-(n-butyl cyclopentadienyl) hafnium dihalide.
 5. The processaccording to claim 1, wherein the catalyst is supported on silica. 6.The process according to claim 1, wherein the catalyst is used togetherwith an aluminiumoxane cocatalyst.
 7. The process according to claim 6,wherein the cocatalyst is selected from the group consisting of methylaluminiumoxane (MAO), tetraisobutyl aminimiumoxane (TIBAO) andhexaisobutyl aluminiumoxane (HIBAO).
 8. The process according to claim1, wherein 30 to 70%, of the ethylene homopolymer or copolymer isproduced at conditions suitable to provide a polymer having a MFR₂ of 10g/10 min or more and 70 to 30%, of the ethylene homopolymer or copolymeris produced at conditions suitable to provide a polymer having a MFR₂ ofless than 5 g/10 min.
 9. The process according to claim 1, whereinessentially no fresh catalyst is added to the reactors other than thefirst reactor.
 10. The process according to claim 1, wherein the processis carried out in a polymerization reactor cascade comprising a loopreactor and a gas phase reactor, in that order.
 11. The processaccording to claim 1, wherein the process is carried out in apolymerization reactor cascade comprising two or more gas phasereactors.
 12. The process according to claim 1, wherein a separationstage is employed between the two reaction stages.
 13. The processaccording to claim 8, wherein 40 to 60% of the ethylene homopolymer orcopolymer is produced at conditions suitable to provide a polymer havingan MFR₂ of 10 g/10 min or more and 60 to 40% of the ethylene homopolymeror copolymer is produced at conditions suitable to provide a polymerhaving an MFR₂ of less than 5 g/10 min.